1. Field of the Invention
The present invention relates to an image forming apparatus for forming images, and more particularly, to an image forming apparatus using an electrophotographic process.
2. Description of the Related Art
In a printing apparatus for printing images by an electrophotographic process, a surface of a drum-type electrophotographic photosensitive member (hereinafter, referred to as photosensitive drum) is uniformly charged to a predetermined potential by a charging unit. In the charging unit, corona charging which is non-contact charging is generally performed. In the corona charging, a high voltage is applied to a thin corona discharge wire to generate corona, and the corona acts on the surface of the photosensitive drum which is to be charged.
In recent years, a contact charging process which is advantageous in terms of a low-voltage process, a low ozone generation amount, and a low cost is becoming mainstream. The contact charging process is a process for bringing, for example, a roller charging member (hereinafter, referred to as charging roller) into contact with the surface of the photosensitive drum and applying a voltage to the charging roller to charge the photosensitive drum. The voltage applied to the charging roller may be only a DC voltage. However, when an AC voltage is applied to alternately generate positive discharging and negative discharging, more uniform charging may be achieved. For example, it is known that an AC voltage having a peak-to-peak voltage (Vpp) which is twice or more larger than a threshold voltage (charge start voltage), at which discharging to the photosensitive drum is started when a DC voltage is applied, is superimposed on the DC voltage to obtain an oscillation voltage to be applied, to thereby uniformly charge the photosensitive member.
When a sinusoidal voltage is applied to the charging roller, the voltage causes a resistive load current to flow into a resistive load between the charging roller and the photosensitive drum, a capacitive load current to flow into a capacitive load between the charging roller and the photosensitive drum, and a discharge current to flow between the charging roller and the photosensitive drum. As a result, the sum of currents flows into the charging roller. As is empirically known, a discharge current amount is desirably maintained to a value equal to or larger than a predetermined value in order to obtain stable charging. Note that, when the discharge current amount becomes equal to or larger than the predetermined value in a high-humidity environment, image defects may occur.
In recent years, high image quality and high stability have been desired, and discharge current control for controlling the discharge current amount has been proposed (see Japanese Patent Application Laid-Open No. 2001-201921).
Image forming apparatuses have been used in a wider range of environments, and increasingly used particularly in a low-temperature and low-humidity environment. In line with this trend, a reduction in cost is strongly desired, and hence the image forming apparatuses are required to be used with a low peak-to-peak voltage (Vpp).
When the discharge current control is employed in the low-temperature and low-humidity environment, a resistance of a charging device increases, and hence a necessary discharge current amount increases. In addition, it is necessary to apply a voltage for computation, and hence the main body of the printing apparatus is required to have a capacity higher than necessary. Therefore, significant power is wasted.